I bought the Mophie Juice Pack Air last June when i got my 3GS, never had any problems with it.
Recently i have seen the Mili come on sale along with the Dexim S7, i have since purchased both of these as i wanted to see the advantages and disadvantages of the three of them.
In short the Mili is the one that i will be keeping, it feels good in the hand, it adds less bulk to the phone than the others, nice clear status indicator, 1200mAh battery which does the job for me (only ever use it when going to the football).
The only downside that i can see is that once you turn it on, there is no turning it off! It powers back to 100% charge and then keeps it there until the Mili runs out. Does not cause me any issues though.
All in all i would say you should go for the Mili.
The Mophie and Dexim are both good but the Mili is my personal preference
Hope that helps in some way.